';by;, I Chem. Mater. 1991,3, 348-355
348
mAh/cm2 was obtained at the C/40 rate, while at 50 OC the utilization was 0.96 mAh/cm2. The Coulombic efficiency during cycling was close to 100%.
0
2 1
"
0.42
0.84
CATHODE CAPACITY.
1.26 MAH/CM~
Figure 16. Constant current discharges of a Li/TiSzcell using 87 MEEP/13 PVP-(LiCIOl)oB electrolyte at C/20 and different temperatures i d = 0.08 mA/cm2;i, = 0.04mA/cm2;voltage limits = 1.6-3.0 V.
overcharge, did not have a deleterious effect on subsequent cell performance. The performance of a Li/TiS2 cell using a mixed polymer electrolyte is illustrated with the MEEP-PVP-(LiC104)o.2s electrolyte, at three different temperatures and 0.05 mA/cm2 (C/20) (Figure 16). At room temperature the cell delivered about 17% (0.14 mAh/cm2) of its theoretical cathode capacity of 0.88 mAh/cm2. At 75 "C, the cathode utilization was 100%. The charging efficiency at room temperature was 859'0,and at the higher temperature it was loo%, indicating slower recharge kinetics at low temperatures. The rate capability of the cell containing MEEP/PVP-(LiC104)o.2swas very much similar to that of the cell described above utilizing MEEP-(LiC104)o.25. Cells utilizing MEEP/PEO-(LiC104)o.13 composite electrolytes exhibited rate capabilities inferior to those described above. For example, a Li/TiS2 cell utilizing 70 wt% MEEP:30 wt% PEO-(LiC104)o,12could not be discharged at room temperature at a practical rate such as C/20 or C/40. However, when it was discharged at 65 "C, about 86% of the theoretical cathode capacity, i.e., 1.2
Conclusions Several relatively simple methods have been developed to prepare dimensionally stable MEEP-based electrolytes. Pure MEEP when complexed with LiAlCl, forms dimensionally stable, free-standing thin films. This contrasta the behavior of MEEP complexes we have prepared with many other commonly used Li salts. Additional methods of dimensional stabilization include blending MEEP-(LiX), with a poly(o1efin oxide) such as PEO or PPO, or forming composites with photopolymers such as PEGDA and PVP. Some of these electrolytes exhibited conductivities close to those of the dimensionally unstable MEEP-(LiX),, indicating that they retain a high degree of plasticity or domains of microfluidity. Transport numbers ranging between 0.4 and 0.5 have been determined for Li+ in these electrolytes, which are very similar to that obtained in other Li+-conductive polymer electrolytes. The dimensionally stabilized MEEP-based electrolytes have demonstrated good electrochemicalstability on a number of metal substrates including Li. The long-term cycling capability of Li/TiS2 secondary solid-state cells fabricated with these electrolytes indicate that they are useful for low-to-moderate rate applications a t ambient temperature. Acknowledgment. This work was supported on Office of Naval Research SBIR Contract "14-874-0857. Registry No. PEO, 25322-68-3; PPO, 25322-69-4; PVP, 9003-39-8; PEGDA, 28158-16-9; LiBF4, 14283-07-9; LiC104, 7791-03-9; LiAsF,, 29935-35-1; LiCF3S03,33454-82-9; LiAlCl,, 14024-11-4; LiN(S02CF3)z, 90076-65-6.
New Poly(carbosilane) Models. 4. Derivatization of Linear Poly[ (methylchlorosilylene)methylene]: Application to the Synthesis of Functional Poly(carbosilanes) Possessing a Poly[ (methylsily1ene)methylenel Backbone E. Bacqu6, J.-P. Pillot,* M. Birot, J. Dunogu&s,*and P. Lapouyade Laboratoire de Chimie Organique et OrganomGtallique (URA 35, CNRS), UniversitB Bordeaux I, 351, Cours de la LibBration, F-33405 Talence Cldex, France
E. Bouillon and R. Pailler Laboratoire des Composites Thermostructuraux, UM 47, CNRS-SEP- UBI, Europarc 3, Avenue Llonard de Vinci, F-33610 Pessac, France Received February 21, 1990. Revised Manuscript Received December 20, 1990 To study the mechanisms of pyrolyeis of poly(carbosiianes),as well as the effect of the structure of these precursors upon properties of the reaulting ceramics, novel organosilicon polymers with welldefined skeletons were prepared. Thus, linear poly(carbosi1anes)formed of exactly alternate silicon atoms and methylene groups were synthesized. Owing to the presence of functional groups on silicon centers, cross-linking of these compounds was then investigated according to chemical routes, by creating disilane, disilazane, disiloxane, and hydrocarbon bridges between different poly(carbosi1anic) sites. These polymers were characterized by physicochemical methods (IR,NMR,GPC, and TGA).
Introduction The need for ceramic composites offering enhanced properties requires both continual improvement of preG o d y r e p o d processes and elaboratiin of new synthtic routes to thew materials. Thus,industrial-de production
of S i c fibers by pyrolysis of poly(carboai1anes) (PCS), according to Y d h ' s route,demonstrafm the considerable interest in organometallic precursors.' (1) Yajima, S. Ceram. Bull. ISM, 62,993.
0S97-4766/9~/2803-0348$02.50/0 0 1991 American Chemical Society
Chem. Mater., Vol. 3, No. 2, 1991 349
New Poly(carbosi1ane) Models. 4
Scheme 11. Reactions of Poly[(methylchlorosilylene)methylene](11)
Scheme I. Synthesis of Linear Poly[(methylchlorosilylene)methylene] (11) and Poly(silapropylene) (111)
\ /
+
2-Si-CI
\ /
Me
Me
I
Me+a-cy*Si-Me
I
CI
I
CI
I1
Me
Me
A
H
+ 2K
2-Si-CI
I
2Na
111
(4 =2 1 ~ 6 0 0 ) However, this route has several inherent drawbacks affecting the silicon carbide fibers as well as the starting poly(carbosilanes). For example, it still remains necessary to improve a number of ceramic fiber properties, such as oxidation resistance and thermomechanical behavior.2 Moreover, although the starting PCS skeletons are known to contain mainly alternate silicon atoms and methylene groups (i.e., Si-substituted silylenemethylene unit?), these compounds exhibit complex polymeric structures that have not yet been fully elucidated. Elemental poly(si1apropylene) units, -(MeHSiCH2),-, are probably present, as inferred from spectroscopic data, but they are part of a strongly branched f r a m e ~ o r k . ~ * ~ On the other hand, very little is known about the mechanisms of the thermal conversion of PCS into silicon carbide. So, it remains difficult to find relationships between the structure of organometallic precursors and preceramic polymers and both the microstructure and properties of the obtained ceramics. To improve our understanding of the thermal conversion of PCS into silicon carbide, it was necessary to establish suitable models. In a previous paper, we reported the conversion of linear poly[(dimethylsily1ene)methylenel, I, into a poly[(methylchlorosilylene)methylene], 11, and subsequent reduction of this last polymer affording poly(silapropylene), I11 (Scheme I). We have shown that this novel poly(carbosi1ane) can be regarded as a convenient linear model of Yajima’s PCS.5p6 More recently, Interrante et al. reported the preparation of the same polymers but with higher molecular weights, using also a ring-opening polymerization route.21 Here, we report the preparation and characterization of a series of functional poly(carbosi1anes) formed from poly[(methylsily1ene)methylenel chains upon derivatization of I1 and 111, as depicted in Schemes I1 and 111.
Experimental Section General Procedures. Solvents: Toluene was dried over sodium by refluxing during several hours and then distilled under argon. THF was distilled under argon just before use from benzophenone-sodium until the appearance of a persistent blue (2) Simon, G.;Bunsell, A. J. Mater. Sci. Lett. 1983,2,80.
(3) Yajima, S . Advanced Fibers and Composites for Eleuoted Temperature; Ahmad, I., Noton, B. R.Eds.; ConferencesProceedings,1979; p 29. (4) Hasegawa, Y.;Okamura, K. J . Mater. Sci. 1986,21, 321. (5) Baqul, E.: Pillot, J.-P.; Birot, M.; Dunogub, J. Macromolecules 1988, 21, 30. (6) Bacqul, E.; Pillot, J.-P.;Birot, M.; Dunogub, J. Macromolecules 1988, 21, 34.
\ /
+
-Si-CI
\ /
2MeflH
t o h ” \ / -Si-Si-
lHF
/
(1)
Ns,nri
\ /
1 \ vs,vi
\ /
WCL,
-Si-NMez
(+2KCI)
(+MeaNH&I)
(2)
(3)
VI
\ /
CH&l2
(+2NaCI)
\
-Si-Si-
-
+ 2MeNH2
-SI-CI
-
-Si-NHMe
(+MeNH3CI)
(4)
VIIS, VIIi
2-
\Si-CI + 3NH3 /
\ /
CHZCC
/ \
-Si-NH-Si-
(+2NH&i)
(5)
VIIIS, v11n
\ /
2-Si-CI
H20
-2HCI
\ /
/ \
-si-0-Si-
(6)
M
I1
Scheme 111. Hydrosilylation Reactions Using Poly(silapropy1ene) (111) in the Presence of Speier’s Catalyst
\ /
+
2-Si-H
\ /
2-Si-H
+
-
\ /
ad.
1,3-butadiene
dhrinybenzene
ad.
-Si+CH2SgSi
/ \
(7)
X
\
tISi4CH2#pi,
(8)
XI
color. Ether was f i t dried over calcium chloride and then distilled under argon from benzophenoneaodium. All of the solvents were stored over on molecular sieves. Apparatus: All the reaction vessels were oven dried (90“C) and purged with argon prior to use. All manipulations involving air-sensitive materials were carried out using an argon vacuum line or a glovebox. Characterization: Molecular weights were determined by gel permeation chromatography (GPC) using five r-Styragel columns (porosity ranges of 100,500, lo3, lo‘, and 105A) and THF as the eluent at a flow rate of 1 mL/min. Molecular weights were obtained from polystyrene standards. The detection system was a Waters Associates R 401 differential refractometer. Proton NMR spectra were obtained on a Hitachi Perkin-Elmer R 24B spectrometer (60 MHz) or a Bruker AC 200 spectrometer, with polymer solutions in CDCl, or C,@, and TMS as the external standard. Infrared spectra were determined on a Perkin-Elmer 1420 spectrometer (in the region 4000-600 cm-3 or a Perkin-Elmer 983 spectrometer (in the region 4000-200 cm-’) with product films respectively between NaCl and Tl(Br1) plates. Thermogravimetricd y s e a were performed on a Perkin-Elmer TGS 2 analyzer (3600 data station, heating rate 5 “C/min; argon flow rate 40 mL/min). Elemental analysis of polymers was carried out by the Service Central d‘halyse du CNRS, B.P. 22, F-69390Vemaison. Reaidual chlorine and oxygen ratios were also measured in a few casea, even if these elements theoretically were not present. Syntheses. Preparation of I: Poly[(dimethylsilylenelmethylene], I, was prepared in quantitative yield by ring-opening polymerization of 1,1,3,3-tetramethyldisila-1,3-cyclobutane ac-
Bacqud et al.
350 Chem. Mater., Vol. 3, No. 2, 1991 cording to the previously reported procedure.' Preparation of I1 Polymer I (75g) and trimethylchlorodane (510 g, large excess) were introduced into a two-neck 500-mL h k equipped with a spinning band column and an argon inlet. This mixture was magnetically stirred at room temperature to dissolve a large amount of polymer I. Anhydrous aluminum trichloride (5.8 g) was then added, and the viscous brown mixture was heated to reflux for several hours until the f i t drops of tetramethylsilane began to condense at the top of the column, resulting in a marked decrease of the reflux temperature from the range 55-57 "C (Le., Me,SiCl boiling point) to 22-28 "C. Sometimes, addition of a few grams of catalyst is necessary to start the reaction. At this time, a significant fluidization of the mixture occurred, and the TMS was slowly distilled as soon as the reaction proceeded. After several hours, the rate of ita formation decreased. When the TMS distillation was completed, the mixture was allowed to cool to room temperature under argon. After filtration under inert atmosphere to remove any solid material, the excess of trimethylchlorosilane was distilled off by using a rotary evaporator, and the crude product carefully devolatilized under high vacuum, affoIding a dark brown viscous polymer (90.5 g, 94% yield). The M,,of I1 was found to vary in the range 2100-2600 according to different runs. Preparation of 111: LiAlH4 (9.1 g, 0.239 mol) and ether (200 mL) were placed into a 500-mL three-necked flask equipped with a dropping funnel, a thermometer well, and a condenser. The flask was placed in an ice bath, and polymer I1 (29.4 g) in solution in ether (150 mL) was then added dropwise with magnetic stirring. At the end of the addition, the mixture was refluxed for 24 h and left at room temperature without stirring to allow precipitation of the solid aluminum salts. The liquid portion was then transferred into another flask, and the ether stripped off under high vacuum. The remaining grey solid was washed twice with hexane (100 mL), and dilute HCI (a few milliliters of 4 N solution) was then slowly added at 0 "C with stirring to the combined liquid fractions (under argon). The organic layer was decanted, washed three times with cold distilled water, and dried (NazS04),and the solvent removed under high vacuum (10-1 Torr) to yield a pale yellow mobile oil (17.5 g, 95% yield). The IR spectrum of this product did not exhibit any shoulders nor broadening of the intense absorption at 1040 cm-' [w(Si-CH,-Si) band]. Thus it can be inferred that completion of the reduction has occurred and therefore that no appreciable amount of Si-H or remaining S i 4 1 bonds have been hydrolyzed. Preparation of IVs and IVi: The apparatus consisted of a IOO-mL, three-neck flask equipped with a magnetic bar, fitted with a dropping funnel, a thermometer well, and a condenser cooled by Decalin at -10 OC, connected to a CaClz column. Xylene (10 mL) and sodium (3.45 g, 50% molar excess) were successively introduced under an argon stream and heated to reflux with vigorous stirring. A solution of poly[(methylchlorosily1ene)methylene], I1 (9.26 g, approximately 0.1 mol (1mol corresponds to 92.5 g of (MeCISiCHz)),residual oxygen
